Annualing separation agent for producing grain-oriented silicon steel with smooth surface and good magnetic property

ABSTRACT

An annealing separator for manufacturing grain-oriented silicon steel with mirror-like surface having good magnetic performance consists of the composition of which is 77˜98 by wt % of Al 2 O 3  powder, 1˜8 by wt % of alkaline earth powder, 1˜15 by wt % of alkali metal chloride and/or alkaline earth metal chloride. The annealing separator of the invention can avoid forming a glass-film undercoating on the surface of the steel sheet during high-temperature annealing, and at the same time, the oxide embedded at near-surface of the sheet is removed by means of corrosive reaction of the chloride, so that a produce with smooth surface and stable magnetic performance is obtained.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturinggrain-oriented silicon steels, especially to an annealing separator formanufacturing grain-oriented silicon steels with mirror-like surfacehaving excellent magnetic performance.

DESCRIPTION OF THE PRIOR ART

Grain-oriented silicon steel shall be subjected to decarburizationannealing protected in a H₂—N₂ atmosphere, after being subjected toprocesses of hot-rolling, normalizing and cold rolling, and accordinglythe rolling stress is relieved and preliminary recrystallization isformed, and meanwhile, wet gas is introduced into a furnace forcontrolling a carbon content in the steel belt below 30 ppm to preventthe final product from magnetic aging. The steel belt will be oxidizedwhen subjected to the decarburization annealing to form an oxide layermainly consisting of SiO₂ and Fe₂SiO₄, which will negatively affect thefollowing decarburization. In the following high-temperature annealingprocess, the oxide layer undergoes chemical reaction with the annealingseparator coated on surfaces of the steel belt, and produces aglass-film undercoating mainly consisting of Mg₂SiO₄. The glass-filmundercoating has the function of preventing the steel belt from bondingand purifying the steel during the high-temperature annealing.

The Mg₂SiO₄ glass-film undercoating on the surface of the grain-orientedsilicon steel has relative high hardness, which results in a relativepoor punching performance of the steel sheet, which is generallythousands of times; and, an embedded combination between the glass-filmundercoating and a body of the steel sheet hinders magnetic domain wallmovement, and increases magnetic hysteresis loss.

In order to improve the punching performance of the grain-orientedsilicon steel and further improve the magnetic performance, Japanesedevelop a grain-oriented silicon steel without the glass-filmundercoating. Japanese patent JP49096920 discloses a method that removesglass-film undercoating on the surface of the grain-oriented siliconsteel by means of acid pickling. However, in order to completely washout the glass-film undercoating with a thickness of 10 μm (including theoxide embedded into the steel sheet), the steel shall be immersed instrong acid for a long period, which results in the problems of highcost, environmental pollution of the agent and the like.

Japanese patent JP05156362A discloses that Al₂O₃ is applied as ahigh-temperature annealing separator. Al₂O₃ does not react with theoxide layer or the body of steel sheet, so that the grain-orientedsilicon steel without the glass film undercoating can be obtaineddirectly. However, the method cannot remove the oxide layer or embeddedoxide formed during decarburizing annealing, which is disadvantage interm of improving the magnetic performance.

To solve this problem, Japanese patent JP2003247024 relates to a methodin which the ratio of PH₂O/PH₂ is controlled to form an atmospherehaving a low degree of oxidizability, thus no Fe based oxide is formed,a separator mainly of Al₂O₃ then is coated to obtain grain-orientedsteel with smooth surface. However, if the degree of oxidizablility istoo low during decarburizing, it will result in the difficulty ofdecarburization. In Japanese patent JP05156364A, after thedecarburization annealing is completed, an oxide layer on the surface ofthe steel sheet is removed by means of acid pickling, and then aseparator mainly of Al₂O₃ is coated.

In U.S. Pat. No. 554,719, MgO+SiO₂ is used as an annealing separator,which forms loose magnesium silicate on surfaces of a steel sheet duringa secondary recrystallization annealing step, then the loose magnesiumsilicate is removed by brushing and washing, so that a product withoutglass-film undercoating is obtained.

In Japanese patent JP2000038615, magnesia and alumina added withchloride are used as an annealing separator, the formed glass filmundercoating is removed by means of interfacial reaction of(2/3)MCl₃+Fe+(3/2)O₂→M₂O₃+FeCl₂↑, so that a product without anyglass-film undercoating is obtained.

JFE, a Japanese company, uses Al₂O₃ and the like, which does not reactwith the surface of the steel sheet, as a high-temperature annealingseparator to directly obtain a grain-oriented silicon steel without anyglass-film undercoating. In such a method, in order to completelyeliminate near-surface oxide impurity of the steel sheet, the dew pointfor decarburizing shall be so strictly controlled that no Fe based oxideis formed on the surface of the steel sheet. However, this willinevitably cause the problem of decarburization and nitridation.

Armco company (now AK company), a US company, uses magnesia, which isadded with SiO₂, as an annealing separator, wherein the loose magnesiumsilicate formed on the steel sheet during a secondary recrystallizationannealing step will benefit in introducing annealing protection gas intointerlayer portion of the steel sheet for purifying the steel. However,generally, such a method cannot completely wash out the magnesiumsilicate on the surface, and cannot completely remove the embedded oxideat the near surface of the iron sheet, either, which restricts theeffect of lowering iron core loss.

NSC, which is a Japanese company, uses magnesia, which is added withchloride, as an annealing separator. However, adding large amount ofchloride will result in certain corrosion to the surface of the steelsheet during a secondary recrystallization annealing, which will affectsurface inhibitor, the secondary recrystallization will be unstable.

TABLE 1 Main composition of the separator fundamental U.S. Pat. 100% byweight of No undercoating reaction No. 3,785,882 Gross Al₂O₃ occursUS554719 (35-85% by weight)MgO + Loose undercoating, which (15-65% byweight) SiO₂ can be easily removed, is formed on the steel sheet surfaceJP08269560 MgO + Cl The undercoating is re- A An amount of Cl added ismoved by interfacial reac- controlled at 0.05-0.5% by tion of (CaC1₂ +Fe weight (1/2)O₂ →CaO + FeC1₂↑)

SUMMARY OF THE INVENTION

The object of the present invention is to provide an annealing separatorfor manufacturing grain-oriented silicon steel with mirror-like surfacehaving good magnetic performance, which can prevent the glass-filmundercoating from forming on the steel sheet, meanwhile the embeddedoxide at the near-surface of sheet can be removed by means of corrosionreaction with the chloride, so that a product with smooth surface andstable magnetic performance can be obtained.

In order to obtain the above-described object, the technical solution ofthe present invention is that:

An annealing separator for manufacturing grain-oriented silicon steelwith mirror-like surface having good magnetic performance consists of acomposition as follows: 77˜98% by weight of Al₂O₃ powder, 1˜8% by weightof alkaline earth metal oxide powder, 1˜15% by weight of alkali metalchloride and/or alkaline earth metal chloride.

Further, the alkaline earth metal oxide comprises BeO, MgO, CaO, SrO, orBaO.

In addition, the alkali metal chloride comprises LiCl, NaCl, KCl, orRbCl.

alkaline earth metal chloride comprises BeCl₂, MgCl₂, CaCl₂, SrCl₂,BaCl₂ or ZnCl₂.

It is found by experiment that it will be effective for removing theoxide layer at the near-surface of sheet by applying a substance thatdoes not react with the oxide layer of the sheet as the annealingseparator during high-temperature annealing, the substance is added witha few amount of alkaline earth metal oxide for introducing moisture nothigher than 2.5 wt %, and a certain amount of chloride is also added, sothat the moisture reacts with the chloride ion contained in the chlorideadditive to form corrosive solution with acidity, which is goodadvantageous for removing oxide layer at the near-surface of sheet.

By adding and stirring water, the annealing separator for thegrain-oriented silicon steel with mirror-like surface of the inventionforms a coating liquid having a certain concentration, then coating onthe surface of the decarburized sheet is carried out. After thecompletion of coating, the product is baked under a temperature nothigher than 300° C. for more than 30 s, so as to expel free moisture inthe separator. At this time, the separator forms a substance havingmicropores, and the main composition of the substance is a mixture ofAl₂O₃, Ca(OH)₂ and one or more kinds of chloride, which has goodpermeability. The primary chemical reaction during the hydrolysis is

CaO+H₂O═Ca(OH)₂  {circle around (1)}

In a preliminary phase of the high-temperature annealing, Ca(OH)₂ issubjected to a decomposition reaction and again produces CaO andreleases moisture when the temperature is higher than 580° C. Thepresence of the moisture at one hand provides some solution, and at theother hand reacts with the chlorine ion to form an acid substance ofHCl, which has a certain corrosion function. Chemical reactions occurredin subsequence during the high annealing are as follows:

Ca(OH)₂═CaO+H₂O  {circle around (2)}

H₂O+Cl⁻

HCl↑+OH⁻{circle around (3)}

HCl in gas phase penetrates through the separator, reacts with the oxidelayer of the sheet, and promote the reaction designated by the chemicalequilibrium {circle around (3)} rightward, so that the reaction occurscontinuously. The reaction between HCl and oxide layer is as follows:

2HCl+FeO═FeCl₂+H₂O↑{circle around (4)}

4HCl+Fe₂SiO₄═2FeCl+SiO₂+2H₂O↑{circle around (5)}

The oxide layer corrupted by HCl degrades to a loose and poroussubstance, the binding force of which with the sheet is reducedsubstantially. By slightly being pickled and brushed afterhigh-temperature annealing, such oxide layer can be easily removed.Thus, the grain-oriented silicon steel with mirror-like surface andsmooth surface can be finally obtained after hot stretching andflattening process.

The glass film undercoating formed during the conventionalhigh-temperature annealing for grain-oriented silicon steel presents arelative high hardness, which will degrade the punching performance ofthe silicon steel sheet, molds will be damaged in some extents duringthe manufacturing. Meanwhile, a pinned structure of the oxide in thebody of sheet hinders the magnetic domain wall movement, which willnegatively affect the magnetic performance. The grain-oriented siliconsteel without undercoating can substantially improves the processabilityof the silicon steel, and the processability thereof can be furtherimproved due to the absence of the pinned structure, so that a productwith extra low iron core loss can be obtained.

Prior to the present invention, patents for obtaining grain-orientedsurface silicon steel mainly relate to MgO and chloride or Al₂O₃. Theformer will result in instability of the magnetic performance, and thelatter cannot remove the embedded oxide formed during decarburizingannealing process. Some one utilizes the Al₂O₃ separator added withchloride, however, the chloride itself needs the assistance of certainmoisture for reacting with the embedded oxide to remove the same.

The invention inventively introduces the alkaline earth metal oxide,based on the water solubility of the alkaline earth metal oxide, themoisture introduced during the high-temperature annealing can becontrolled easily. Such a method is easy, and can stably obtainexcellent grain-oriented silicon steel products. The apparatus concernedis conventional apparatus for producing grain-oriented steel, which hasexcellent practicability and spreadability, which features goodexpectation of popularizing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional optical photograph of a steel sheet of comparativeexample 1 (separator: MgO 65Wt % plus SiO₂ 35 Wt %).

FIG. 2 is a sectional optical photograph of a steel sheet of comparativeexample 2 (separator: MgO 90Wt % plus CaCl₂ Wt %).

FIG. 3 is a sectional optical photograph of a steel sheet of comparativeexample 3 (separator: Al₂O₃ 100 Wt %).

FIG. 4 is a sectional optical photograph of a steel sheet of anembodiment of the invention (the separator: Al₂O₃ 86 Wt % plus CaO 4 Wt% plus MgCl₂ 10 Wt %).

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in connection withembodiments.

A 500 Kg-vacuum furnace is used for steel-smelting, the chemicalcomposition of a steel blank is (in Wt %): 0.045% by weight of C, 3.25%by weight of Si, 0.006% by weight of S, 0.027% by weight of Als, 0.006%by weight of N, 0.15% by weight of Cu, 0.012% by weight of Mn and abalance consisting of Fe and inevitable impurities. After being heatedunder 1150° C., the blank is hot rolled to form a hot rolled sheet witha thickness of 2.6 mm. The hot rolled sheet is normalized and annealedfor 1 minutes, and then is pickled and cold rolled to form the sheetwith a final thickness of 0.285 mm. The cold rolled sheet undergoesdecarburizing annealing treatment under 835° C. for 120 s, so there aretwo levels of the oxygen content on the surface: 0.8 and 1.6 g/m²; afterthe process of nitriding, the nitrogen content of the steel sheet is 250ppm. The decarburized and annealed sheet is coated by the annealingseparator (the material proportion is shown in Table 2), after beingwound, the sheet undergoes high-temperature annealing at 1200° C., whichtemperature is held for 20 hours, in the protective atmosphere of drynitrogen and hydrogen, then the sheet is coated with an insulationcoating, stretched and flattened, and annealed after unwound.

TABLE 2 (% by weight) alkali metal alkaline earth chloride/alkalinedescription Al₂O₃ metal oxide earth chloride Embodiment 1 98 CaO 1 MgCl₂1 Embodiment 2 86 CaO 4 MgCl₂ 10 Embodiment 3 77 CaO 8 MgCl₂ 15Embodiment 4 86 BeO 4 LiCl 10 Embodiment 5 86 MgO 4 NaCl 10 Embodiment 686 SrO 4 KCl 10 Embodiment 7 86 BaO 4 RbCl 10 Embodiment 8 86 MgO 4BeCl2 10 Embodiment 9 86 SrO 4 CaCl2 10 Embodiment 10 86 BaO 4 SrCl2 10Embodiment 11 86 CaO 4 BaCl2 10 Embodiment 12 86 CaO 4 ZnCl2 10comparative 65 parts of MgO + 35 parts of SiO₂ example 1 comparative 90parts of MgO + 10 parts of CaCl₂ example 2 comparative Al₂O₃ 100 partsexample 3

The average values of the electromagnetic performance of the resultedproducts and the surface qualities thereof are shown in table 3.

TABLE 3 Electromagnetic Surface performance oxygen P_(17/50), Separatorcontent B₈, T W/kg Surface appearance Embodiment 1 0.8 g/m² 1.897 0.753Smooth surface, no undercoating 1.6 g/m² 1.905 0.745 Smooth surface, noundercoating Embodiment 2 0.8 g/m² 1.891 0.783 Smooth surface, noundercoating 1.6 g/m² 1.897 0.774 Smooth surface, no undercoatingEmbodiment 3 0.8 g/m² 1.899 0.735 Smooth surface, no undercoating 1.6g/m² 1.903 0.734 Smooth surface, no undercoating Embodiment 4 0.8 g/m²1.888 0.779 Smooth surface, no undercoating 1.6 g/m² 1.897 0.748 Smoothsurface, no undercoating Embodiment 5 0.8 g/m² 1.889 0.776 Smoothsurface, no undercoating 1.6 g/m² 1.895 0.773 Smooth surface, noundercoating Embodiment 6 0.8 g/m² 1.900 0.769 Smooth surface, noundercoating 1.6 g/m² 1.900 0.743 Smooth surface, no undercoatingEmbodiment 7 0.8 g/m² 1.890 0.782 Smooth surface, no undercoating 1.6g/m² 1.903 0.775 Smooth surface, no undercoating Embodiment 8 0.8 g/m²1.895 0.768 Smooth surface, no undercoating 1.6 g/m² 1.893 0.760 Smoothsurface, no undercoating Embodiment 9 0.8 g/m² 1.899 0.772 Smoothsurface, no undercoating 1.6 g/m² 1.903 0.769 Smooth surface, noundercoating Embodiment 10 0.8 g/m² 1.887 0.766 Smooth surface, noundercoating 1.6 g/m² 1.890 0.760 Smooth surface, no undercoatingEmbodiment 11 0.8 g/m² 1.897 0.771 Smooth surface, no undercoating 1.6g/m² 1.910 0.743 Smooth surface, no undercoating Embodiment 12 0.8 g/m²1.887 0.775 Smooth surface, no undercoating 1.6 g/m² 1.899 0.762 Smoothsurface, no undercoating comparative 0.8 g/m² 1.927 0.705 The surfaceincludes example 1 partial undercoating 1.6 g/m² 1.921 0.720 The surfaceincludes complete undercoating comparative 0.8 g/m² 1.825 0.997 Thesurface includes example 2 partial undercoating 1.6 g/m² 1.857 0.897 Thesurface includes partial undercoating comparative 0.8 g/m² 1.865 0.903The surface includes example 3 partial undercoating 1.6 g/m² 1.847 0.937The surface includes partial undercoating

It can be seen from FIGS. 1-4 and Table 3 that there is few oxideresidual existing on the surface of the silicon steel sheet coated withthe separator of the invention, and the magnetic performance of thesteel sheet are good. Thus, it can be seen that the grain-oriented steelsheet with mirror-like surface having good magnetic performance can bemanufactured by the effective finish process on the surface of thegrain-oriented silicon steel in the present invention.

On one hand, the high-temperature annealing separator of the presentinvention effectively purifies the steel and prevents coils of the steelfrom binding, and on the other hand, the present invention provides acorrosive atmosphere during the annealing with high temperature toremove the oxide layer at near-surface, so that grain-oriented siliconsteel with mirror-like surface having good magnetic performance can beproduced.

1. An annealing separator for manufacturing a grain-oriented siliconsteel with good magnetic performance, which consists of a composition asfollows: 77˜98% by weight of Al₂O₃ powder; 1˜8% by weight of alkalineearth metal oxide powder; 1˜15% by weight of alkali metal chlorideand/or alkaline earth metal chloride.
 2. The annealing separator formanufacturing a grain-oriented silicon steel with good magneticperformance according to claim 1, wherein the alkaline earth metal oxidecomprises BeO, MgO, CaO, SrO or BaO.
 3. The annealing separator formanufacturing a grain-oriented silicon steel with good magneticperformance according to claim 1, wherein the alkali metal chloridecomprises LiCl, NaCl, KCl or RbCl.
 4. The annealing separator formanufacturing a grain-oriented silicon steel with good magneticperformance according to claim 1, wherein the alkaline earth metalchloride comprises BeCl₂, MgCl₂, CaCl₂, SrCl₂, BaCl₂ or ZnCl₂.